Nondestructive characterization of biological tissue often employs light-tissue interaction. More particularly, the refractive index of tissue is often a valuable component of tissue analysis. Recent studies have shown, for example, that refractive index variations have potential diagnostic capability in distinguishing cancerous breast tissue. Studies have also emphasized the importance of refractive index variations due to changes in biochemical environment. For instance, it has been shown that in common medical practice oxygen saturation modulates the refractive index of hemoglobin, thus affecting the optical measurement of blood oxygenation.
Several methods have been proposed to measure refractive index in biological tissue, but all have specific limitations. Both optical coherence tomography (OCT) and confocal scanning laser microscopy (CSLM), for example, are used for non-destructive subsurface optical imaging of tissue, but suffer from depth-dependent distortions caused by inhomogeneities of the refractive index in the sample and of the medium surrounding the sample. Bifocal optical coherence refractometry (BOCR), for example, which tomographically maps the refractive index based on an interferometric measurement of a distance between two foci that are scanned depth-wise through a sample, relies on measurement of light backscattered by the sample, but is limited by the non-uniformity of such scattering. Methods of measuring refractive index by confocal microscopy (CM), using the difference (between the nominal focus position and actual focus position) induced by index of refraction variation, suffers, particularly, from low resolution in the direction of optical axis of the confocal microscope. Moreover, most interferometric techniques are known to be extremely susceptible to mechanical vibrations.
To date, understanding of the refractive index properties of cellular structures is, in some respects, limited by the challenges confronting its direct measurement in situ, and further improvement of imaging capabilities is highly desirable.